1. Field of the Invention
The present invention is directed to a recording material for the inkjet printing process having a substrate and an ink receiving layer which is applied on at least one side of the substrate, the ink receiving layer containing an inorganic pigment and an ink fixative comprising at least two substances, the first substance being a polyamine epichlorohydrin and the second substance being a polyvalent metal salt. The invention is further directed to a process for recording by discontinuous inkjet printing which uses the novel recording material.
2. Description of the Prior Art
Reaction products which are produced on the basis of amine compounds and epihalohydrins as auxiliaries for the ink receiving layers of inkjet recording materials are known. In order to improve the water fastness of printed images which are generated by means of inkjet printers and which should be distinguished by excellent printing quality, JP-A-11 277 888, for example, proposes an auxiliary with a linear cationic resin that is formed as reaction product of an amine component containing a secondary amine and another amine component containing at least two amino groups and an epihalohydrin.
JP-A-11 277 887 is likewise directed to an auxiliary which increases water fastness and which has a structural formula of the linear cationic resin that is different than that in the above-cited reference and has a comparable effect. According to the description, however, this structural formula is expanded in order to reduce ink bleeding in images printed by inkjet printers. In both of the references cited above, the indicated viscosity of the respective 20-percent aqueous solutions containing the proposed auxiliary is between 50 and 500 m Pa*s (B-type; 30° C.).
JP-A-10 152 544 has the object of providing an inkjet recording paper which allows only slight ink bleeding in printed images generated by aqueous inks and, beyond this, enables a high color concentration and excellent water fastness of the printed images. This object is supposed to be met through an addition to the coating compound in the form of a branched cationic resin presenting a reaction product of ammonia and at least one, preferably primary, secondary or tertiary amine and an epihalohydrin. Further, preferable amines are polyalkylene polyamine and alkanolamine. The Brookfield viscosity of the 10-percent aqueous solution of this cationic resin is 1 to 30 m Pa*s (60 rpm/25° C.).
With the aim of providing a production process for inkjet recording papers which has only low bleeding of ink droplets of aqueous ink and which enables printed images with high color density and excellent water fastness, JP-A-09 240 139 proposes application of a branched cationic resin which is formed as a reaction product of a polyalkylene polyamine and an epihalohydrin and further, as the case may be, with an aliphatic amine component. The Brookfield viscosity of a 10-percent aqueous solution containing the proposed cationic resin is indicated as 30 m Pa*s (60 rpm/25).
The teaching to be gathered from the texts cited above is the suitable selection of the cationic resins to be used as auxiliaries in medium-molecular linear form or low-molecular branched form. However, these texts do not indicate possible quantity ratios of the cationic resins to the pigments present in the ink receiving layers, nor do they indicate that it is advantageous to combine the cationic resins to be used as auxiliaries with metal salts to obtain improved characteristics for recording materials to be used in inkjet printing methods.
A polyamide polyamine epichlorohydrin as ink fixative in the ink receiving layer of an inkjet recording material is known from JP-A-09 099 630. However, neither its precise structure nor its molecular weight is disclosed. Amorphous silicon dioxide is proposed as pigment; its particle size is indicated only in a very general way as averaging in the range of 6 to 13 μm.
According to EP-A-0 914 962, outstanding inkjet recording characteristics and superior printability in offset printing are achieved in a recording material due to its surface being especially well bonded; further, this recording material is distinguished by a high degree of water fastness. It is stated in this text that this goal is achieved by a linear cationic resin in the ink receiving layer. Dimethylamine epichlorohydrin polycondensation products, among others, are mentioned as examples of this cationic resin. In addition to the cationic resin, the ink receiving layer further contains binders and, as the case may be, pigments. The text does not indicate any advantageous characteristics of the pigments in the ink receiving layer. Also, the description in this text does not disclose any teaching regarding the mixture ratio of ink fixative to pigment. A ratio of 1:10 is disclosed by way of example, while other examples disclose pigment-free ink receiving layers.
Finally, EP-A-0 602 326 discloses a quaternary salt of a linear dimethylamine epichlorohydrin adduct with a degree of polymerization between 2 and 2000 as ink fixative in the recording layer of an inkjet recording paper. In addition to improved ink fixing, color distortion is prevented in the applied printed images through combination with a (meth)acrylamide diallylamine copolymer. Insofar as they are used at all in the recording layers disclosed in this text, inorganic and organic pigments considered to be suitable are those having a particle size in the range of 4 μm. There is no indication in the text about the advantageous combination of the dimethylamine epichlorohydrin adduct with polyvalent metal salts as ink fixative.
Also, the use of divalent or polyvalent metal salts in the recording layer of inkjet recording materials is known and is proposed without combining with other agents improving ink fixing in DE-A-25 33 957 and DE-A-24 01 866, for example. However, using metal salts alone or predominantly as ink fixatives in inkjet recording layers has the fundamental disadvantage that a true-color reproduction of applied ink print images is rarely achieved.
EP-A-1 01 016 545 discloses a recording material for inkjet printing in which a mat finish, greater ink absorption, image quality, water-tightness, light fastness and ink transfer, as well as resistance to ink bleeding, are to be achieved by means of a binder mixture for the image receiving layer containing polyethylene glycol and polyvinyl alcohol in a defined ratio. The image receiving layer can contain polyvalent metal salts and a compound from the group of polymeric quaternary ammonium compounds or base polymers such as poly(dimethylaminoethyl) methacrylate, polyalkylene polyamine and condensation products thereof with dicyanodiamide and amino-epichlorohydrin polycondensates. The structure and molecular weight of these compounds are not disclosed. For the purpose of increasing the density of open porous structures, the image receiving layer contains colloidal oxides such as colloidal silicon dioxide or silicon dioxide modified with aluminum oxide without disclosing advantageous quantity ratios.
Finally, DE 34 33 528 C2 proposes a recording material for inkjet printing with a carrier containing, at least in the surface region, a water-soluble metal salt of a metal with an ionic valence of 2 to 4 and a cationic organic material chosen from alkylamine salts, quaternary ammonium salts, polyamines and basic latices from the group of polyamine latices and alkylammonium latices. Examples of polyamines mentioned therein include polyethylamine epichlorohydrin. However, the text does not contain further indications of its especially suitable representatives nor does it refer to any particular arrangements of specific representatives of polyethylamine epichlorohydrin. The text also does not indicate any teaching of advantageous characteristics of the pigments to be employed, e.g., with respect to particle size.
For a long time, pigment-based recording inks were rarely used in place of inks based on organic dyes, particularly acidic azo dyes. Problems with this type of recording ink have to do with the low light fastness of the organic dyes along with the problem of fading and discoloration of the printed images, which could be solved heretofore only unsatisfactorily by means of UV absorbers in the recording inks and by means of color stabilizers in the inkjet recording materials. The pigment-based recording inks that have been used in the meantime have substantially greater light stability than the organic dye-based inks mentioned above. However, the pigment-based recording inks have the problem of ink bleeding, by which is meant that directly adjoining printed patterns of different colors run into one another directly after the printing process. The recording material according to the invention was developed for use with pigment-based recording inks and significantly reduces the problem of ink bleeding.
There are basically two different processes of droplet generation in inkjet printing. The continuous process provides an inkjet which is ejected under pressure from a nozzle and which separates into very small droplets at a certain distance from the nozzle due to surface tension. The droplets are electrically charged and, by means of deflecting plates which are electronically controlled by the electrical field, are subsequently either deflected into a collecting vessel or placed on the recording material depending on the printed image to be generated.
In the discontinuous drop-on-demand process, as it is called, the ink droplets, depending on the printed image to be generated, are generated and ejected from a nozzle only when an image point is to be generated on the recording material. One type of drop-on-demand printer uses the piezoelectric effect in which an electrically controlled piezo-element separates an ink droplet from the reservoir of recording ink and ejects this droplet from a nozzle. In contrast, bubblejet printers use an electrically controlled heating element which allows very small quantities of aqueous ink to form in a steam bubble. The resulting steam pressure ejects the droplet.